WASHINGTON — President Barack Obama has the upper hand in the fierce struggle over immigration now taking shape, with a veto pen ready to kill any Republican move to reverse his executive order, Democrats united behind him and GOP congressional leaders desperate to squelch talk of a government shutdown or even impeachment.

With the public favoring changes in the current immigration system, the Republicans’ best short-term response appears to be purely rhetorical: that the president is granting amnesty to millions, and exceeding his constitutional authority in the process. Beyond that, their hopes of reversing his policies appear to be either a years-long lawsuit or the 2016 presidential election.

Neither of those is likely to satisfy the tea party adherents in Congress — or the Republican presidential contenders vying for support among party activists who will play an outsized role in early primaries and caucuses just over a year away.

“We alone, I say it openly, we the Senate are waiting in our duty to stop this lawless administration and its unconstitutional amnesty,” said one of them, Sen. Ted Cruz of Texas. In remarks on the Senate floor, according to his office, he was channeling Cicero, the ancient Roman orator.

In a portion of the oration that Cruz did not mention, Cicero referred to a Roman Senate decree calling for a conspirator against the Roman republic “to be put to death this instant.”