FILE - In this Jan. 29, 2019, file photo, Senate Intelligence Committee Chairman Sen. Richard Burr, R-N.C. speaks during a Senate Intelligence Committee hearing on Capitol Hill in Washington.Retiring House and Senate Republicans are a natural group to watch for defectors as Democrats’ impeachment inquiry of President Donald Trump builds steam. (AP Photo/Jose Luis Magana, File)

FILE - In this Jan. 29, 2019, file photo, Senate Intelligence Committee Chairman Sen. Richard Burr, R-N.C. speaks during a Senate Intelligence Committee hearing on Capitol Hill in Washington.Retiring House and Senate Republicans are a natural group to watch for defectors as Democrats’ impeachment inquiry of President Donald Trump builds steam. (AP Photo/Jose Luis Magana, File)

WASHINGTON (AP) — Top Republicans eager for a united GOP front will be eyeing retiring lawmakers for signs of cracking as Democrats’ impeachment inquiry of President Donald Trump heats up. So far, there’s no indication that the retirees are about to crack.

Party leaders got a positive signal this week from departing Sen. Lamar Alexander, R-Tenn., who’s respected by both parties as an old-school defender of Senate prerogatives and is considered a gauge of moderate GOP sentiment. Alexander, 79, said that while it was “inappropriate” for Trump to have prodded foreign governments to investigate his political opponents, impeachment “would be a mistake.”

It’s too early for many Republicans to take irrevocable positions on impeachment. No one knows what fresh evidence might emerge as investigators probe Trump’s effort to pressure Ukrainian leaders to provide damaging information about former Vice President Joe Biden, a Democratic presidential contender.

But though most retirees are leaving politics and face less pressure to defend Trump, they’re acting much like their GOP colleagues who are sticking around. They’ve downplayed the case against Trump, criticized the impeachment investigation, been vague or said nothing.

A look at the retirees themselves and at President Bill Clinton’s impeachment two decades ago helps explain why.

The GOP’s four senators and 17 House members who aren’t seeking reelection next year are largely loyalists with little history of bucking party leaders. Political consultants and academics consider past voting records and ideology key indicators of whether lame-duck lawmakers will abandon their party.

“They can bark as much as they want” with their rhetoric, said Michael Romano, a political science professor at Virginia’s Shenandoah University who’s studied lame-duck lawmakers. “Whether or not that translates into an actual vote is kind of mitigated by ideology, a lot.”

During Clinton’s impeachment, just two House Democrats and six Republicans who were retiring, and none from the Senate, crossed party lines. The overwhelming number of defectors ran for reelection and won.

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In today’s hyperpartisan climate, if the House impeaches Trump, it seems hard to envision 20 Senate Republicans joining all Democrats for the two-thirds majority required to remove Trump from office. Some retiring Republicans might be reluctant to cast a futile vote against Trump after a lifetime of party loyalty, while others might view it as a way to burnish their reputations for independence.

“You’ve just got to decide where the evidence lies and where you want your legacy to be,” said Republican pollster Jon McHenry.

Senate Intelligence Committee Chairman Richard Burr, R-N.C., has commented little on impeachment and is considered worth watching. He’s said he won’t seek reelection in 2022, and his panel is conducting its own investigation of the whistleblower complaint against Trump.

The most closely observed House retiree is probably moderate Texas Rep. Will Hurd, 42. A former undercover CIA officer, he’s opposed hard-line Trump immigration policies and was considered a rising Republican star before retiring. He’s expressed doubt that Trump’s actions are impeachable but said on CBS’ “Face the Nation” that he supports hearings “to get to the bottom of this.”

Departing Rep. John Shimkus, R-Ill., told St. Louis radio station KMOX on Thursday that was removing his name from “the ‘I support Donald Trump’ list” because Trump pulled U.S. troops from Syria, prompting a Turkish attack on U.S. Kurdish allies.

Shimkus later said he was withdrawing as an official Trump campaign supporter, leaving unaddressed how that might reflect his view on impeachment.

Among other Republicans leaving Congress, Michigan Rep. Paul Mitchell called the impeachment push a “circus” in an interview but said he would “protect and defend the Constitution.” Wisconsin Rep. Jim Sensenbrenner, 76, said Congress is “wasting its time” because there’s insufficient public support for removing Trump.

Senate Majority Leader McConnell has already signaled he will protect Trump. The Kentucky Republican, who’s seeking reelection next year, says in an online campaign ad that stopping impeachment requires “a Senate majority, with me as the majority leader.”

When the GOP-run House voted to authorize its Clinton impeachment inquiry in October 1998, just two of the 31 Democrats who joined all Republicans in voting “yes” were retiring.

Just one of those retirees joined four other Democrats and voted that December to impeach Clinton. He was Pennsylvania Rep. Paul McHale, who backed three of the four articles of impeachment. The House approved two of them.

McHale, a former Marine and 48 at the time, said in a recent interview that while he was retiring, he contemplated seeking office again and still faced political pressure. He said he voted for impeachment because Clinton had lied about his affair with White House intern Monica Lewinsky.

“I lost a lot of friends because of that vote,” said McHale, who never returned to elective office.

Six lame-duck House Republicans each opposed one article of impeachment against Clinton. On the four articles, overall House GOP defections ranged from five to 81.

During the February 1999 Senate impeachment trial, “I tried to hold them in line,” former Senate Majority Leader Trent Lott, R-Miss., said of GOP senators in a recent interview.

Even so, 10 Republicans voted against one or both impeachment articles. None were lame ducks.

No Democrats crossed party lines. Both articles were rejected.

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Associated Press Writer Andrew Taylor contributed to this report.