Republicans are reluctant to defend President Trump over his dealings with Ukraine because they believe the White House's defense lacks credibility, or they are concerned they could be contradicted by new discoveries, reported The Washington Post.

For the president to be impeached, Democrats will need the support of the Republican-controlled Senate.

GOP support for the president has so far been muted. And only three Republican senators have come out and criticised President Trump for calling for Ukraine, and then China, to investigate domestic political rival Joe Biden and his son, Hunter Biden.

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Republican lawmakers are reluctant to defend President Trump amid an impeachment inquiry into his dealings with Ukraine, reported The Washington Post.

According to conversations with 21 lawmakers, aides and advisers, GOP lawmakers won't champion the White House's defense of the president's conduct and "fully parrot White House talking points because they believe they lack credibility or fret they could be contradicted by new discoveries," the Post said.

One former Republican official told the Post that the president's own conduct was also an issue weighing on Republicans.

"Everyone is getting a little shaky at this point," Brendan Buck, who was counselor to former House speaker Paul Ryan, told the publication. "Members have gotten out on a limb with this president many times only to have it be cut off by the president. They know he's erratic, and this is a completely unsteady and developing situation."

Another issue reportedly of concern to lawmakers is the enormous support the president has from the party's grassroots supporters, where according to a recent Fox News poll 89% approve of the job he is doing.

The White House did not immediately respond to a request for comment on the report.

Trump currently faces one of the most serious crises of his presidency after House Democrats launched an impeachment inquiry into his having pressured Ukraine to investigate Joe Biden, a leading contender for the 2020 Democratic presidential nomination, and his son, Hunter Biden.

The White House has alleged a "deep state" plot to unseat the president, but Republican lawmakers have been silent or relatively muted in their defense of the president's conduct.

For the Democrats' impeachment bid to succeed Democrats will need the support of two-thirds of the Senate, which is currently controlled by the Republicans. Attention is focusing on the question of whether Republicans will choose to break with Trump and impeach him, or will choose to back him and reject an impeachment bill.

Mitt Romney speaks to journalists on January 24, 2019. Melina Mara/The Washington Post via Getty Images

So far, only three Republican senators have criticized Trump's calls for foreign powers to investigate his political rival: Sen. Susan Collins of Maine, Sen. Mitt Romney of Utah, and Sen. Ben Sasse of Nebraska.

In comments broadcast by CNN Sunday Colin Powell, a moderate Republican who served as secretary of state in the administration of President George W Bush, called on the GOP to "get a grip on itself."

"They need to get a grip, and when they see things that aren't right they need to say something about it. Because our foreign policy is a shambles right now, in my humble judgement. And I see things happening that are hard to understand," Powell said.

"Republican leaders and members of the Congress, both Senate and the House, are holding back because they're terrified of what will happen to any one of them if they speak out," Powell told a Jefferson Series panel.