WASHINGTON — After two grueling election cycles, Guy Cecil, the brains behind the Democrats’ improbable Senate showings in 2010 and 2012, was expected to set aside his political combat boots for tasseled loafers and a sinecure somewhere in this city that pays handsomely for success.

Then his old boss, Senator Michael Bennet of Colorado, reluctantly took the helm of the Democratic Senatorial Campaign Committee, looking at another brutal map for Democrats eager to stay in control of Congress’s upper chamber. He had one demand: Keep Guy Cecil aboard.

“It was critically important that Guy stay in the job,” said Mr. Bennet, now reunited with Mr. Cecil, the former chief of staff who spent the last months of his 2010 campaign sleeping in the senator’s Colorado basement. “He is just excellent at what he does.”

Mr. Cecil’s return as executive director of the committee is notable in a city accustomed to political consultants cashing in for big money “downtown” — at lobbying firms and with influence peddlers off Capitol Hill. In 2010, Mr. Cecil helped engineer Mr. Bennet’s successful defense of his seat, one of the unexpected wins that kept Democrats in control of the Senate even as the party suffered a historic defeat in the House. Most assumed Democrats would lose the Senate as the 2012 season began. With Mr. Cecil directing forces, the party gained two seats.