Sen. Rob Portman of Ohio is one of the few Republicans lawmakers to openly rebuke President Donald Trump for asking the new Ukrainian president in late July to investigate one of Trump's major Democratic rivals, former vice president Joe Biden.

"The President should not have raised the Biden issue on that call, period," Portman told The Columbus Dispatch on Monday. "It is not appropriate for a president to engage a foreign government in an investigation of a political opponent."

Portman's views are a relative rarity among the 252 Republican members of Congress. For the president to be impeached and removed from office, 20 out of 53 Republican senators would need to join with the Democrats and vote to remove him.

For Portman, that's a step too far for now.

Portman said despite his differences with Trump, who recently urged both Ukraine and China to investigate Biden, he did not view Trump's conduct as an impeachable offense. Trump so far has commanded overwhelming Republican backing as he battles against the House Democrats' impeachment inquiry that was sparked by the phone call between the U.S. and Ukrainian presidents.

However, a small but growing number of Republicans are favoring an investigation as more details of the scandal emerge, according to a VOA review.

Former Secretary of State Colin Powell, who served in Republican President George W. Bush's administration, recently criticized the state of U.S. foreign policy and blamed Republicans for being "terrified" of criticizing Trump.

Powell said during a televised lecture moderated by CNN columnist Fareed Zakaria that "Republican leaders and members of the Congress . . . are holding back because they're terrified of what will happen [to] any one of them if they speak out." What they fear, he said, was losing their primary elections if Trump forces came after them. The four-star general suggested that losing a primary would not be "such a disaster."

In all, about 16 prominent Republicans including Portman and Powell have publicly raised concern about Trump's conduct or defended the House Democrats' right to seek answers to questions about Trump's efforts to enlist foreign countries to investigate Biden and his son, Hunter -- who did business in Ukraine and China during his father's tenure as vice president. Here is a summary of the 14 other Republicans who have spoken out.

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Congressman Mark Amodei: "Let's put it through the process and see what happens."

The Nevada House member initially suggested that the impeachment inquiry is justified, but later pulled back. In a call with the Nevada Independent, Amodei stated he was "a big fan of oversight, so let's let the committees get to work and see where it goes." Later Amodei emphasized that "In no way, shape, or form, did I indicate support for impeachment." He said that he would base his vote on whether he found "credible evidence" that Trump "broke a specific law."

Senator Ben Sasse: "Americans don't look to Chinese commies for the truth."

The Nebraska senator wrote to the Omaha World-Herald after Trump suggested China investigate Joe Biden for corruption. "If the Biden kid broke laws by selling his name to Beijing, that's a matter for American courts, not communist tyrants running torture camps," Sasse wrote. The senator called the House Democrats' impeachment inquiry a "partisan clown show." In contrast, he said the Senate inquiry is "working to follow the facts one step at a time." Sasse previously said the whistleblower complaint against Trump that triggered the impeachment probe contained "real troubling things" and that Republicans "ought not just circle the wagons." However, he has not made a judgment on whether Trump should be impeached.

Senator Susan Collins: "The president made a big mistake by asking China to get involved in investigating a political opponent."

The Maine senator took issue with Trump's comments on the White House lawn that China should investigate Biden for corruption. Collins said she had no comment on the current evidence for the impeachment inquiry. She said she hopes the impeachment inquiry will "be done with the seriousness that any impeachment proceeding deserves." Collins said she was preparing for the likelihood that the House would send articles of impeachment to the Senate. Collins told the Bangor Daily News that she plans to act as a juror as she did in the impeachment trial of President Bill Clinton in 1998.

Congressman Brian Fitzpatrick: Law Enforcement should look into the case and report to Congress

The Pennsylvania House member does not support the House's impeachment inquiry. Instead he suggests allowing law enforcement to evaluate the case. "Whether or not law enforcement matters and investigations should be initiated or closed are decisions that should be made by law enforcement and law enforcement alone, not by politicians," Fitzpatrick said in a statement. Fitzpatrick was an FBI agent assigned to anti-corruption efforts in Ukraine in 2015 at the time Joe Biden was vice president and his son was working in Ukraine.

Senator Mitt Romney: "The President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

The Utah senator and former presidential candidate issued a statement on Twitter after President Trump suggested China could investigate Joe Biden for corruption. The senator tweeted: "When the only American citizen President Trump singles out for China's investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated. President Trump struck back at the senator in a series of expletive tweets saying Romney "choked" in the 2012 presidential election and tagging one tweet '#IMPEACHMITTROMNEY.'

Congressman Will Hurd: "We need to fully investigate all of the allegations addressed in the letter."

The Texas congressman said the House should investigate the allegations in the whistleblower's report. On CBS' 60 Minutes, Hurd emphasized he wanted to understand "the motivations and intentions" of those involved in the phone call. "What I want to do is understand the truth," said Hurd. He is on the committee leading the investigation into the whistleblowers complaints. Hurd called the impeachment inquiry "wordplay" used by House Speaker Nancy Pelosi to "placate some of the extreme wings of her party."

Congressman John Curtis: Says he has the "utmost confidence in the investigative tools Congress has at its disposal."

The Utah House member released an official statement saying that he is "closely monitoring" the formal inquiry and that he was pleased that Trump released the transcript of his call with Ukrainian President Volodymyr Zelenskiy.

Senator Chuck Grassley: "No one should be making judgments or pronouncements without hearing from the whistleblower first."

The Iowa senator issued a statement saying the whistleblower who revealed Trump's call should be heard out and protected. Grassley did not offer an opinion on whether Trump should be impeached. Instead he said that "uninformed speculation wielded by politicians or media commentators as a partisan weapon is counterproductive and doesn't serve the country." The senator said that media reports on the whistleblowers identity "don't serve the public interest." Grassley is the chairman and co-founder of the Senate Whistleblower Protection Caucus.

Congressman Troy Balderson: "At this moment we don't have all the facts."

The Ohio House member told Spectrum News that he believed in "full transparency," in the impeachment inquiry. Balderson said the allegations against the president are "serious and concerning." The representative said he looks forward to "reviewing all available information so Congress may address the situation based on the facts presented to us." Balderson narrowly won his district's special election thank to an endorsement from Trump and former governor John Kasich, a moderate Republican.

Congressman Steve Stivers: "These are very serious allegations, and I'll be monitoring the situation closely."

The Ohio House member's spokeswoman told The Columbus Dispatch that Stivers "has concerns about the president's call with the Ukrainian president, but has been encouraged by the amount of information that has been disclosed in the past couple of weeks."

Jeff Flake: "35 Republican Senators" would convict Trump if the vote were a secret ballot.

The former Arizona senator suggested that many GOP senators want to break away from Trump but are concerned about backlash from voters. Flake made the comment in response to political consultant Mike Murphy's statement on MSNBC that he had been told by an anonymous Republican senator that 30 Republican senators would impeach Trump if their vote was secret. "There's a lot of fear of what it means to go against the president," said Flake, a critic of the president, "but most Republican senators would not like to be dealing with this for another year or another five years."

Trump's Republican primary challengers have also weighed in on the inquiry, varying from supporting an investigation into potential wrongdoings to suggesting the president could be convicted of treason.

Mark Sanford: "Very troubling charges" against Trump.

The former South Carolina Governor and 2020 presidential candidate appeared on CNN's State of the Union to address the House's impeachment inquiry. Sanford suggested a congressional censure of Trump might be more appropriate, but did not outright say impeachment proceedings would be wrong. He favors Congress leading an investigation into potential wrong doing.

Joe Walsh: "Donald Trump is a traitor."

The former Illinois House member and 2020 presidential candidate appeared alongside Sanford on CNN's State of the Union to address the House's impeachment inquiry. "He [Trump] stood on the White House lawn ... and told two additional foreign governments to interfere in our election. That alone is impeachable." said Walsh. The presidential candidate said if he was still in Congress, where he served one term, he would vote to impeach Trump. Walsh said he did not consider the president's actions to be treasonous.

Bill Weld: "It's treason, pure and simple."

The former Massachusetts governor and 2020 presidential candidate, appeared on MSNBC's "Morning Joe," and suggested that Trump's phone call to the Ukraine president could go beyond impeachment. "Talk about pressuring a foreign country to interfere with and control a U.S. election, it couldn't be clearer, and that's not just undermining democratic institutions," Weld said. "That is treason. It's treason, pure and simple. And the penalty for treason under the U.S. code is death. "That's the only penalty."