A bipartisan group of more than 40 ex-senators are calling for the current members of the upper chamber to join together to defend democracy, saying that the U.S. is entering a "dangerous period" as the investigation into Russian election interference nears its conclusion.

"We are on the eve of the conclusion of special counsel Robert S. Mueller III’s investigation and the House’s commencement of investigations of the president and his administration," the 44 senators write in an op-ed addressed to current senators for The Washington Post. "The likely convergence of these two events will occur at a time when simmering regional conflicts and global power confrontations continue to threaten our security, economy and geopolitical stability.

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"It is a time, like other critical junctures in our history, when our nation must engage at every level with strategic precision and the hand of both the president and the Senate."

The former senators go on to write that the U.S. is at an "inflection point" in which the core principles of democracy and national security interests "are at stake."

The letter notes that at other times in U.S. history, the Senate has been the body to stand up for democracy during times when it was threatened.

"Today is once again such a time," the senators write.

"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 by ensuring that partisanship or self-interest not replace national interest."

The letter was published as Mueller's investigation into Russian election interference and possible collusion between the Trump campaign and the Kremlin appears to be zeroing in on some of the president's closest allies.

Meanwhile, House Democrats have vowed to open their own probes into the president regarding a range of matters once they officially take over the majority leadership in the House in January.

Trump has repeatedly derided the investigation into Russian election interference, often referring to it as a "witch hunt."