WASHINGTON (Reuters) – Immigration and domestic security, key themes in Donald Trump’s successful campaign, will likely dominate two U.S. Senate hearings on Tuesday as lawmakers begin several days of questioning the president-elect’s Cabinet nominees.

First to appear before lawmakers will be Trump’s pick for attorney general, Republican Senator Jeff Sessions of Alabama. Sessions, a close ally of Trump, helped shape his pro-enforcement, anti-amnesty policy on illegal immigration.

Next will be John Kelly, a retired four-star Marine general tapped to head the Department of Homeland Security. In earlier congressional testimony, Kelly characterized inadequate policing of the U.S.-Mexico border as a national security threat.

Both men will face questions from Democrats and Republicans seeking specifics on Trump’s plans following his Jan. 20 inauguration to crack down on illegal immigration – an issue central to his explosion onto the political scene, but on which he has since wavered in some ways.

On the campaign trail, Trump promised to build a wall on the U.S.-Mexico border, force Mexico to pay for it and deport 11 million immigrants living illegally in the United States.

But since he was elected president on Nov. 8, the New York businessman has said part of the wall could be a fence, Congress should fund it with the expectation that Mexico will repay U.S. taxpayers, and that he will focus on deporting immigrants with criminal records and later decide what to do with others.

Both Sessions and Kelly will be major players in immigration policy. In addition to counterterrorism, the Homeland Security secretary oversees immigration enforcement and has discretion over which categories of immigrants are arrested and deported.

AMERICA’S TOP PROSECUTOR

The attorney general is the nation’s top prosecutor and legal adviser to the president. As head of the Justice Department, the attorney general also oversees the immigration court system that decides whether immigrants are deported or granted asylum or some other kind of protection.

“Sessions was a close adviser to Trump. … They’re going to ask, ‘How are you going to use your position to further the president’s agenda?'” said Elizabeth Taylor, a former staffer for the Senate Judiciary Committee who advised Republicans during Eric Holder’s nomination to be Democratic President Barack Obama’s first attorney general.

“But,” Taylor added, “historically, attorney general nominees are also asked if they’re willing to stand up to the president.”

In 2015, Republicans held up the nomination of Loretta Lynch, the current attorney general, for 166 days, longer than any nominee in 30 years, over her support for Obama’s executive actions on immigration.

Sessions, 70, and Kelly, 66, are widely expected to be confirmed by the Republican-dominated Senate, but their hearings could be contentious.

Sessions, who has represented the deeply conservative Southern state of Alabama for 20 years, has a long, consistent record of opposing legislation that provides a path to citizenship for immigrants. He has also been a close ally of groups seeking to restrict legal immigration by placing limits on visas used by companies to hire foreign workers.

Roy Beck, president and founder of NumbersUSA, which advocates a reduction in illegal and legal immigration, endorsed Sessions in a letter to Senate Judiciary Committee Chairman Charles Grassley.

“Sessions always has made immigration decisions based on protecting the economic interests of hard-working women and men whose incomes and very jobs have been threatened by the desire of various business lobbies to increase the foreign labor competition in their occupations,” Beck wrote in a Jan. 3 letter.

Civil liberties groups have raised concerns about Sessions’ record on immigration and other positions, including government surveillance, civil rights and marijuana legalization.

He was denied confirmation to a federal judgeship in 1986 after allegations emerged that he made racist remarks, including testimony that he called an African-American prosecutor “boy,” an allegation Sessions denied.

The American Civil Liberties Union’s legal director will testify at Sessions’ confirmation hearing and “raise significant, serious questions about his hostility to civil rights and civil liberties,” the organization said in a statement. The group said it is making an exception to its longstanding policy of not interfering with federal nominations in this case.

On Monday, a group of civil liberties and internet freedom groups sent a letter to the Senate Judiciary Committee describing Sessions as a “leading proponent of expanding the government’s surveillance authority of ordinary Americans.”

Sessions has long condemned marijuana use, which has been legalized for recreational use in eight U.S. states and the District of Columbia but remains banned by federal law. As attorney general, Sessions would have the power to intervene in states that are not in compliance with federal law. He has also opposed attempts to reduce prison sentences for nonviolent drug offenders.

(Reporting by Julia Edwards Ainsley; additional reporting by Dustin Volz and Ian Simpson; Editing by Kevin Drawbaugh and Jonathan Oatis)

IMAGE: Donald Trump sits with U.S. Senator Jeff Sessions (R-AL) at Trump Tower in Manhattan, New York, U.S., October 7, 2016. REUTERS/Mike Segar/File Photo