(CNN) Forty-four former Republican and Democratic US senators penned an op-ed for The Washington Post that warns "we are entering a dangerous period" and urges current and future senators to be "steadfast and zealous guardians of our democracy."

The former senators write in the op-ed they feel an "obligation to speak up about serious challenges to the rule of law, the Constitution, our governing institutions and our national security."

"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 senators write. "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."

The US is at an "inflection point" the senators write, "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."

During their time in office, the former senators write, "at times we were allies and at other times opponents, but never enemies."

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