The new Republican-led Senate will try to assert itself on foreign policy, putting pressure on President Obama in the one area where second-term presidents often enjoy considerable leeway.
Although the president has the constitutional authority to execute foreign policy, the GOP Congress will seek to leverage its power where it can through legislation and the power of the purse, as well as through rhetoric.
"A nuclear-armed Iran would pose the biggest long-term threat to American and allied national security, so Congress needs to immediately vote on the Menendez-Kirk legislation, a bipartisan bill to stop a nuclear Iran that already has 60 supporters in the Senate," said Kirk, who worked on the bill with Sen. Robert Menendez (D-N.J.).
David Rothkopf, CEO and executive editor of Foreign Policy, said Republicans could "draw a line in the sand" on a nuclear pact with Iran, with some of more hawkish members of the GOP arguing "it is not a good deal for America or its allies.”
Such a move could "doom" what would be a "signature accomplishment" for the president, Rothkopf added.
Republicans will also likely seek to impose harsher sanctions on Russia for its aggression toward Ukraine, and to ratchet up the pressure on Obama to provide Ukrainian forces with weapons — something the administration has been unwilling to do out of concern it could provoke Russia further.
"We renew our call for the administration to impose the toughest possible sanctions against Russia, provide long-requested weapons to Ukraine, and bolster further NATO preparedness in eastern Europe," said Sen. Kelly AyotteKelly AyotteTen rumored Trump Cabinet picks who didn't get a job Sasse, Perdue join Armed Services Committee Avid pilot among GOP senators joining Transportation committee MORE (R-N.H.) in a joint statement this week with Sen. Lindsey Graham (R-S.C.).
Most foreign policy experts say the president and his advisers will remain the prime drivers of strategy against the Islamic State in Iraq and Syria (ISIS). But even there, Republicans can seek to exert influence via legislation, hearings or public criticism.
Republicans could also simply try to stymie the president on issues that have been central to his foreign policy agenda since his first run for the presidency in 2008 — namely, winding down U.S. involvement in Iraq and Afghanistan, and closing the Guantánamo Bay detention facility.
Although the president has already embarked upon a new military mission in Iraq to defeat ISIS, he still hopes to withdraw all U.S. troops from Afghanistan by the end of 2016, a move which Republicans have indicated they oppose.
They also staunchly oppose allowing President Obama to close the Guantánamo Bay detention facility.
"It's better to constrain a president's action than tell him to do something he doesn't want to do,” said Richard Fontaine, president of the Center for a New American Security.
As chairman of the Senate Armed Services Committee, Sen. John McCainJohn McCainSchumer, Cardin to introduce legislation on Russia sanctions Graham says he will vote for Tillerson McCain will vote for Tillerson MORE (R-Ariz.) would control the crafting of the Senate's annual defense policy bill. McCain is one of the administration's top critics on foreign policy, and has long argued for more aggressive stances on several issues, including ISIS, Russia and Iran.
Although the president has the power to veto legislation, experts said Republicans could challenge the president in ways that he might find difficult to surmount.
For example, Congress could insert restrictions into spending bills that the president would find difficult to veto, said American University law professor Stephen Vladeck.
“Unlike policy bills, where the president can simply sit back and veto legislation that he finds offensive, when it comes to appropriations bills, it’s not necessarily obvious that these are bills that the president is going to be in position to refuse to sign,” said Vladeck.
Sen. Thad CochranThad CochranGOP senators voice misgivings about short-term spending bill Trump's wrong to pick Bannon or Sessions for anything Bottom Line MORE (R-Miss.), a Navy veteran, is expected to control the Senate Appropriations Defense Subcommittee and backs robust defense spending, an aggressive stance toward ISIS and Iran, and opposes closing Guantánamo.
Fontaine said besides vetoing legislation, the president has two other options to continue to executive foreign policy — "try to do as much as he can through executive action” or compromise with Republicans.
Rothkopf said such compromises could include rolling back defense cuts, reaching a Trans-Pacific Partnership trade deal, and giving a green light to the Keystone XL oil pipeline, since there is now likely a "veto-proof majority" in the Senate of Republicans and Democrats on that question.
Council on Foreign Relations Director of Studies James Lindsay said the president would still have significant authority in foreign policy, since there are some authorities Congress cannot impinge upon, including negotiating treaties.
The president can also nominate people to crucial foreign posts, but the Senate has final approval and could jam up the process.
Republicans could send a “message to the world that it's going to be very hard for the president to get his ideas through Congress,” Rothkopf said.
Whenever the president deals with foreign leaders or attempts to initiate a joint effort of any kind, "there's going to be a question in the mind of those leaders on whether he can follow through," which weakens the administration, he said.
Republicans could also hurt the administration's policies they disagree with by calling for investigations.
The GOP could use its newfound subpoena power to investigate "situations where there is a hint of scandal or impropriety, or where they want to get behind the scenes," such as the administration’s response to crises like ISIS and Ebola, Rothkopf said.
John Bolton, who served as a controversial U.S. ambassador to the United Nations during the presidency of George W. Bush, said Republicans should try to formulate their own vision for the future.
“I would hope that since Republicans would have control of both the House and Senate, that this is a real opportunity to have a broader review in the Foreign Relations committees, in the Armed Services committees, and in the intel committees of what America's role in the world should be" said Bolton, whose political action committee and super-PAC donated to Republican candidates during the midterm election.
"Having control of the committee structures ... allows a really thorough airing of all these issues," he added.
This story was updated at 11:48 a.m.