Sen. Charles J. Colgan, the longest-serving member of the Virginia Senate, said Tuesday that he will run for a 10th term in November, a move that boosts Democrats’ chances of retaining their thin majority in the legislature’s upper chamber.

Colgan (D-Prince William), who is chairman of the Senate Finance Committee, had said a year ago that he would probably retire at the end of this year. “I’m not Strom Thurmond,” said Colgan, who was ailing from prostate cancer.

But on Tuesday, Colgan said he has beaten his cancer and is in excellent health. He had been swayed, he added, by fellow Democrats and supporters who urged him to run for another four-year term.

“I’m going to give it one more shot,” said Colgan, 84. “I talked to my family and a lot of my friends — I’ve gotten literally dozens of phone calls. I walked into McDonald’s and a man said, ‘I’ve been a Republican all my life, and you’re my favorite Democrat. Don’t you dare quit.’ ”

Democrats hold a 22-18 majority in the Senate, the party’s only power base in Richmond, with the governor’s mansion and the House of Delegates controlled by Republicans.

Although Colgan’s seat has been leaning Republican in recent years, Democrats think the well-respected octogenarian could fairly easily win reelection.

Democrats had worked to dilute growing GOP strength in Colgan’s district when they drew its new boundaries in the recently concluded redistricting process — an attempt to entice him into the race. If he had retired, the area would have been vulnerable for a GOP pickup.

“This is a huge morale booster,” said Sen. Majority Leader Richard L. Saslaw (D-Fairfax). “Chuck is unquestionably the most popular elected official in that county. . . . He’s really been Mr. Prince William, and he’s been it for almost 50 years.”

Colgan, who helped found regional carrier Colgan Air but no longer owns the company, had twice decided to step down, but he was persuaded by the Democratic governors Mark Warner in 2003 and Timothy M. Kaine in 2007 to run again.

Marquee issue

Battle for control of the 40-member Senate will be the marquee issue in the November election. Republicans will mount an aggressive effort to win back the chamber, particularly in conservative southern Virginia, where they think Democratic senators Phillip P. Puckett (Russell) and William Roscoe Rey­nolds (Franklin) are vulnerable.

Republicans will also challenge two Democrats elected in 2007, Sen. Ralph S. Northam (Norfolk) and John C. Miller (Newport News).

Democrats won control of the body in 2007, the first time since 1999.

Also hotly contested will be a new Senate seat drawn in Loudoun and Prince William during the redistricting process to accommodate growth in the Washington’s outer suburbs. Three Republicans have filed to run, and Democrats say they expect to have a candidate soon.

“Obviously, an incumbent seeking reelection makes it a steeper climb to win the seat,” Senate Republican caucus spokesman Jeff Ryer said of Colgan’s district. But, he added: “There are many other seats in play. It’s not as though this decision will determine the makeup of the Virginia Senate.”

No Republican has yet filed to challenge Colgan. Del. Jackson H. Miller (R-Manassas) was thought to be considering a run if Colgan retired. He said Tuesday that he has not ruled out a challenge but suggested other Republicans might step forward instead.

Candor and civility

In the Senate, Colgan has been known for his candor and civility. A moderate Democrat and committed Catholic, he sometimes crosses party lines to vote with Republicans, particularly on abortion issues. He was one of two Democrats who voted with all 18 Senate Republicans this year to require that abortion clinics be regulated as hospitals. Lt. Gov. Bill Bolling (R) broke a tie vote, sending the bill to the governor.

Colgan was one of five Democrats to support a measure last year that prohibits government or employers from requiring Virginians to maintain health insurance, a law being used by Attorney General Ken Cuccinelli II (R) to challenge the constitutionality of the federal health-care law.

But Colgan, a veteran of the Army Air Corps and the Air Force, has been a stalwart Democratic vote on fiscal issues. He has pushed to spend more state dollars on education, health care and public safety and has supported unsuccessful Demo­cratic efforts to raise taxes to pay for transportation improvements.

“He’s been such a great representative for the region, and across most of the county, people know him very. very well,” said Sen. Mary Margaret Whipple (Arlington), who chairs the Demo­cratic caucus. “We probably could win it without Chuck — but this is better.”