For more than two years, a breach has been opening between President Obama and the foreign policy establishment of the Democratic Party. Last week, as Russia pressed a new offensive in Ukraine and the Senate debated sanctions on Iran, it cracked open a little wider.

First came the introduction in the Senate, and lopsided passage by the Banking Committee, of a bill that would place new sanctions on Iran if no agreement limits its nuclear program by June. Though fiercely opposed by Obama, the measure, co-sponsored by Sen. Robert Menendez (N.J.), the senior Democrat on the Foreign Relations Committee, had won the express support of 13 other Democratic senators by the end of the week. A letter signed by Menendez plus nine of them pledged to delay a final floor vote until March 24, the deadline set by the administration for finalizing the framework of a ­bargain.

While that postponement avoided an immediate confrontation with Obama, the larger message of the senators was clear: They are “deeply skeptical,” said the letter, that Obama will obtain adequate concessions from Tehran — despite what has been an increasingly single-minded diplomatic push.

At week’s end came another de facto vote of no confidence: a report by eight foreign policy luminaries, due to be formally released Monday, saying the president should “immediately change” his policy of refusing to supply Ukraine with weapons to defend its besieged eastern provinces. “Washington,” it said bluntly, has “not devoted sufficient attention to the threat posed by Russia and its implications for Western security. This must change.”

This rebuke was signed by Michèle Flournoy, the deputy defense secretary in Obama’s first term; Ivo Daalder, his first-term NATO ambassador; and Strobe Talbott, a former deputy secretary of state who is president of the deep-blue Brookings Institution. It expanded on legislation calling for arms sales to Ukraine that passed Congress last month with sponsors that included Menendez and seven other Senate Democrats, including Carl Levin (Mich.), the ranking member of the Armed Services Committee, and Richard Durbin (Ill.), the second-ranking Senate Democrat.

The Democratic rebellion against Obama’s policies began with Syria and Obama’s refusal to provide support to rebels battling the regime of Bashar al-Assad. Obama’s rejection in 2012 of a proposal by then-Secretary of State Hillary Clinton and then-CIA Director David Petraeus to train and arm the rebels alienated a wide swath of the Democratic foreign policy mainstream — including Levin, who has campaigned for creating a no-fly zone in northern Syria, and Rep. Adam Smith (Wash.), the ranking Democrat on the House Armed Services Committee. Smith called for a formal Pentagon training program for Syrian rebels a year before Obama finally agreed to it.

The dissension now encompasses Ukraine, where Obama is seen as having been too slow and limited in his response to Russia’s gross violation of international treaties guaranteeing European borders, and, even more so, Iran — where a growing number of Democrats worry that Obama is offering too-generous terms while failing to challenge Iran’s conventional aggression in the Middle East, at the expense of Israel and traditional U.S. Arab allies.

The rift is not particularly ideological; this is not a case of Clinton and center-right allies facing off against Obama and the party’s more liberal wing, as during the 2008 presidential primaries. Levin was a staunch opponent of the Iraq war, as was Daalder. Smith has been a strong Obama supporter. One of the co-sponsors of the arms-for-Ukraine legislation was Sen. Sherrod Brown (D-Ohio), one of the most liberal members of the caucus.

Rather, it has more to do with Obama’s extreme caution in responding to international challenges, as in Syria and Ukraine; and his radical faith that longstanding U.S. adversaries can be converted into strategic partners. Obama’s reaction to the Iraq war, which has been to avoid even indirect U.S. military engagement in all other international conflicts, has had the effect of creating common cause between anti-Iraq doves like Levin and Daalder and relative hawks such as Clinton.

Similarly, Obama’s belief that Iran, if granted its nuclear infrastructure and what it regards as its rightful place in the Middle East, will play a constructive role in ending the wars in Syria and Iraq looks scary not just to senators like Menendez but also to longtime Obama allies like Virginia’s Tim Kaine, who said in a recent Senate hearing that he had “a series of very significant concerns” about the emerging deal with Tehran.

So far, there’s little sign that the Democratic criticism has had much effect on Obama. He’s still balking at significant intervention in Syria, still refusing even defensive weapons for Ukraine, still intently focused on cutting a deal with Iran. If and when that comes, the showdown with his own party may begin.

Read more from Jackson Diehl’s archive, follow him on Twitter or subscribe to his updates on Facebook.