NEW YORK — A bipartisan group of 57 senators have signed a letter this week calling on President Barack Obama to take a more aggressive stance against the Iranian regime in order to stop its nuclear program.

Noting that “Democrats and Republicans in Congress have set aside their differences and worked together on a bipartisan basis with your Administration to put in place the most stringent sanctions ever against Iran,” the letter urges Obama “to reiterate your readiness to take military action against Iran if it continues its efforts to acquire a nuclear weapon.”

Initiated by senators Kelly Ayotte (R-NH), Robert Menendez (D-NJ) and Joe Lieberman (I-CT), the letter openly calls for the Obama administration to undermine the current authoritarian regime in the country and to support more liberal democratic elements.

“We urge you to expand America’s outreach and support to the Iranian people and support of the cause of human rights and democracy in Iran,” the senators wrote, adding, “the current government of the Islamic Republic of Iran will eventually end up in the ash heap of history, not because of the efforts of the United States, but because of the desire of the Iranian people to enjoy the basic freedoms that are their universal right, and that many of their neighbors increasingly are demanding.”

The letter comes against the backdrop of the renewal next month of international diplomatic discussions with the Iranian regime over its nuclear program, and the growing conviction among many observers that 2013 will be a decisive year for Western efforts to prevent Iran from obtaining nuclear weapons.

It also comes amidst speculation in Washington that Obama is looking to appoint former Nebraska senator Chuck Hagel as the next Secretary of Defense, succeeding the retiring Leon Panetta. Hagel, a respected Republican known for working well with Democrats, is in the minority opinion among American political leaders on the Iranian issue, having argued in the past against a military strike opposing the Iranian nuclear program and speculating that the United States might be able to live with a nuclear-armed Iran.

The Iranian government “has continued to press forward with its nuclear program,” the senators charged. “It has quintupled its stockpile of low enriched uranium since 2009,” “taken a significant step closer to possessing weapons-grade uranium by enriching up to 20 percent,” and “raced towards completion of its hardened Fordow enrichment facility, more than doubling the number of centrifuges installed there just since the summer of this year.”

The senators urged Obama “to work with our European and Middle Eastern allies to demonstrate to the Iranians that a credible and capable multilateral coalition exists that would support a military strike if, in the end, this is unfortunately necessary.”

According to the senators, “there is no national security challenge that is more urgent and essential to resolve during your second term.”