(CNN) If Congressional Republicans are waiting for a clear signal of discontent from the party base before asserting more independence from President Trump, they may be waiting for a long time.

That's not because Trump has some distinctly powerful hold on his party rank and file. It's because through the history of modern polling it has been rare for a president to face widespread defections from voters in his own party.

That's been especially true for Republicans. In Gallup presidential job approval polling dating back to the 1940s, only one Republican president has ever drawn support from less than half of his party's partisans: Richard Nixon -- twice in the final months before the Watergate scandal forced his resignation in August 1974. And even Nixon remained safely above 50 percent with Republicans through most of his long ordeal.

This history challenges the assumption among many Republican operatives that President Trump's high approval ratings among GOP voters creates a unique barrier that prevents party leaders from questioning him more forcefully, whatever their private misgivings about his handling of the presidency and the steady drip of revelations about his campaign's interaction with Russia.

In fact, it's never been easy for Members of Congress to challenge a president of their own party. When Senators and House Members have done it, they have almost always been leading, rather than following, public opinion, especially among their own voters. And yet in earlier generations party leaders have taken precisely that risk when they concluded the national interest demanded it.

At this point in his tenure, Trump's standing is strong in his own party, but not uniquely so, and weaker with the public overall than almost any recent president. In the most recent Gallup weekly average , Trump draws positive job approval ratings from 85 percent of Republicans, but under 40 percent of all Americans. (This week's ABC News/Washington Post national poll produced similar results.)

Except for Bill Clinton, who stumbled through a tumultuous first year, every other recent president since Ronald Reagan had comparable approval numbers at this point among those in their own party, and much stronger numbers with the public overall. In early July 2009, Gallup recorded Barack Obama's approval rating among Democrats at 92%; at roughly this point, George W. Bush stood at 88% with Republicans, George H.W. Bush at 86% and Reagan at 85%. In sharp contrast to Trump, each of them drew positive job ratings from a majority of the broader public. Clinton's numbers six months in were slightly below Trump's in his own party, and above them with the overall public.

Perhaps even more important, Trump's political position now is no stronger, and in some ways clearly weaker, than that of other presidents when they were confronted by Senators and House Members from their own party at key moments in the past half century.

LBJ and Vietnam

pivotal in sparking broader skepticism about the war. One such example came in 1966 when Democratic Senator William Fulbright of Arkansas, the chairman of the Senate Foreign Relations Committee, defied President Lyndon B. Johnson to begin a landmark series of televised hearings questioning the progress of the Vietnam War. When Fulbright convened the first hearing in February, Johnson's approval rating in Gallup polling stood at 74% among Democrats and 61% with the general public. The hearings, as historian Robert Mann wrote in A Grand Delusion , his epic history of Vietnam and Congress, "incensed" LBJ, who "resolved to do whatever he could to undermine the committee's proceedings." Yet Fulbright pressed forward and historians now consider the hearingspivotal in sparking broader skepticism about the war.

Nixon and Watergate

A demonstration outside the Whitehouse in support of the impeachment of Richard Nixon in 1974.

Although Watergate eventually swept away Nixon's support, Republican Senators stood against an even stronger current when they supported the initial steps to fully investigate the scandal. The Senate voted unanimously in February 1973 to create the Select Committee on Presidential Campaign Activities -- better known as the Watergate Committee -- led by Democratic chairman Sam Ervin and Republican vice-chairman Howard Baker.

At that point, Nixon's approval rating stood at 91% among Republicans and 65% with the public overall, each better than Trump's standing today. Even when the committee held its first public hearing in May 1973, Nixon in Gallup still drew support from 73% of Republicans, not much less than Trump today, and 44% percent of the public overall, better than Trump now.

As the scandal metastasized, Nixon's approval rating eroded, both with Republicans and the general public. Yet even in Nixon's "final days" during the summer of 1974, the GOP leaders who pressed him to resign were surmounting significant resistance from their own voters. Nixon's approval rating among Republicans in Gallup never fell below 48% and stood at 50% in the final survey before his resignation. Through most of 1974, about 70 percent of rank-and-file Republicans consistently told Gallup that Nixon should not be impeached and compelled to leave office. Even in August 1974, when 57% of all Americans said Nixon should be forced from office, 59% of Republicans still disagreed.

And yet all three Senate Republicans on the Watergate Committee signed its damning June 1974 final report, and senior party officials, including former presidential nominee Barry Goldwater and the Republican leaders in the House (John J. Rhodes) and Senate (Hugh Scott), joined in creating the irresistible pressure for Nixon to resign soon thereafter.

Reagan and the Iran-Contra scandal

Reagan speaking to the press during Iran-Contra hearings.

Few presidents have been more iconic for voters in their own party than Ronald Reagan. But just as during Vietnam and Watergate, many of his own party's legislators demanded a full accounting after the Iran-Contra scandal erupted in late 1986.

In January 1987, virtually every Republican in both the House and Senate voted with Democrats to create bipartisan select committees to investigate the complex arms for hostages scheme. At that point, Reagan's approval rating in Gallup polling stood at about 80% among Republicans and nearly 50% with the general public -- again a stronger overall position than Trump now.

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The two chambers quickly merged their dual investigations into a bipartisan joint committee on Iran-Contra, which released a highly critical report in November 1987. That process split along party lines more than the Watergate investigation, with the committee's six House Republicans all refusing to sign the tough majority report. But three Senate Republicans did sign that report -- even though Reagan's approval rating around that time in Gallup remained at about 80% among Republicans and 51% overall.

Congressional Republicans may genuinely believe Trump's actions, particularly on Russia, do not deserve more systematic scrutiny and questioning than Congress has provided so far. But this history suggests it's a dodge for them to argue that his hold on their party's voters essentially ties their hands. The bookended defections from centrist and conservative Senate Republicans that on Monday night again at least temporarily derailed the White House-backed push to repeal the Affordable Care Act suggests the Congressional GOP deference to Trump has its limits. Yet it remains unclear whether the health care revolt will stand as an exception-or the beginning of a more independent posture toward a president whose volatility has exasperated, if not alarmed, many Republicans in Congress. One thing is already clear: When the times demanded it, earlier generations of Congressional leaders have stood up to demand answers and accountability from presidents whose grip was at least as strong on their own party -- and much stronger on the public overall.