WASHINGTON — The Iran nuclear deal is taking center stage in the presidential campaign as the agreement gains steam on Capitol Hill and lawmakers prepare for what could be the most consequential foreign policy vote of their careers.

Democratic presidential candidate Hillary Rodham Clinton plans a speech today in Washington to support the deal. Republican presidential candidates Donald Trump and Ted Cruz and other conservatives, including former Alaska Gov. Sarah Palin, are headlining an anti-deal rally on the lawn of the Capitol.

The House, returning from summer recess, is expected to vote on a resolution of disapproval this week. House Democratic leader Nancy Pelosi of California, who is working hard to bolster support for the deal, has invited ambassadors from the other five nations in the agreement to talk today with House Democrats. They also are expected to meet with Secretary of State John Kerry, a lead negotiator of the accord.

Joined by military veterans, several dozen House Democrats gathered Tuesday evening on the steps of the Capitol to show their support for the deal with Iran. Sounding a familiar theme, they said the agreement is about verification, not trust.

“Tonight we stand, members of Congress, on the steps of the Capitol, as members totally committed to preventing a nuclear-armed Iran,” Pelosi said.