After more than a week of vote counting and only periodic election calls, the shape and the resulting stakes of the next Senate have finally come into sharp relief: Party control will be determined by a pair of runoff elections in Georgia in January, and who wins those races will have profound implications on President-elect Joseph R. Biden Jr.’s policy agenda.

Senate race calls made on Wednesday formalized outcomes that were widely expected, with Senators Dan Sullivan of Alaska and Thom Tillis of North Carolina, both Republicans, holding onto their seats and giving their party a 50-to-48 advantage.

If Republicans win either Georgia race on Jan. 5, they will maintain control of the chamber. Democrats must win both seats in the traditionally conservative state to leverage Vice President-elect Kamala Harris’s tiebreaking vote and take control.

If Democrats do win both races, close aides to Mr. Biden and economists who helped advise his campaign say the president-elect will try to push through a large stimulus plan for the flagging economic recovery — most likely along the lines of the $2.2 trillion that House Democrats approved this fall.

His stimulus plan under such a scenario would include hundreds of billions of dollars for state and local governments that have lost tax revenue amid the pandemic recession, extended unemployment benefits for people who lost jobs during the crisis and a new round of aid for small businesses.

Mr. Biden’s team is also developing a government employment program — called the Public Health Jobs Corps — that would put 100,000 Americans to work on virus testing and contact tracing.

A narrow majority in the Senate would also give Mr. Biden the chance to push through tax increases on corporations and the rich to fund ambitious plans like rebuilding roads and bridges, speeding the transition to carbon-free energy and helping Americans afford health care.

But if Republicans win at least one Georgia seat, Mr. Biden will most likely need to settle for a wave of executive actions that would bring more incremental progress toward his policy goals while trying to cut compromise deals with Senator Mitch McConnell of Kentucky, the majority leader.

Tax increases, even for the ultrarich, would almost certainly be off the table, as would expanding the Affordable Care Act to give Americans the ability to buy into a government insurance program like Medicare. Mr. Biden would continue to push for infrastructure and health care bills, economists around him say, but he would be unlikely to win support for his full agenda in those areas.

Courtesy – The New York Times