President Donald Trump (AP)

With Special Counsel Robert Mueller "on the eve of the conclusion" of his work, 44 former U.S. senators called on the upper chamber's current members to place the national interest over "partisanship or self-interest."

"[W]e are entering a dangerous period, and we feel an obligation to speak up about serious challenges to the rule of law, the Constitution, our governing institutions and our national security," the senators wrote Monday night in the bipartisan Washington Post opinion column.

They added:

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Former GOP Sen. John Warner (AP)

"Regardless of party affiliation, ideological leanings or geography, as former members of this great body, we urge current and future senators to be steadfast and zealous guardians of our democracy."

The op-ed comes just days after court filings by Mueller's team implicated President Donald Trump in a campaign-finance violation, a felony. Other court filings and news reports have led legal analysts to theorize that Mueller soon will indict members of Trump's family -- and possibly even Trump himself, if the acting U.S. attorney general allows it -- for various activities during the 2016 campaign, including illegal contacts with Russian agents who sought to sow chaos in the U.S. and tilt the election to Trump.

President Trump, a Republican, has repeatedly insisted that Mueller's investigation is a "witch hunt" and that there was "no collusion" between his campaign and Russia.

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Darron CummingsFormer GOP Sen. Richard Lugar (AP)

The retired senators signing the op-ed include former Democratic presidential candidates Bill Bradley, Tom Harkin, Gary Hart, Bob Kerrey and John Kerry, and former Democratic vice-presidential candidate Joe Lieberman.

Among the Republicans who signed it are former Defense secretaries William Cohen and Chuck Hagel, former Senate Foreign Relations Committee chairman Richard Lugar, former Senate majority whip Alan Simpson and former Senate Armed Services Committee chairman John W. Warner.

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Former Trump campaign chairman Paul Manafort (AP)

The senators, worried that the U.S. is in the midst of a slow-rolling constitutional crisis, wrote that the country is "at an inflection point in which the foundational principles of our democracy and our national security interests are at stake, and the rule of law and the ability of our institutions to function freely and independently must be upheld."

Robert Mueller, a lifelong Republican, is a former FBI director. His special-council investigation has already resulted in nearly 200 criminal charges, including a fraud conviction of Trump's former campaign chairman Paul Manafort.

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-- Douglas Perry

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