Massachusetts Democratic Rep. Stephen F. Lynch will announce his decision on a Senate run next week, but has been setting the groundwork for a bid — from fundraising to political maneuvering — should he decide to get in.

Lynch spokesman Scott Ferson told CQ Roll Call the congressman is doing everything he would need to do to mount a Senate bid next week and will make a final announcement on his decision then, when Sen. John Kerry is expected to be confirmed as secretary of State.

Lynch on Friday denied local media reports that he had made up his mind to challenge Rep. Edward J. Markey, telling the Boston Herald that he is still weighing what he would do. “I realize that time is short, but I still think we have a shot,” Lynch said to the paper.

Kerry is expected to be confirmed as secretary of State next week and immediately resign his Senate office. Democratic Gov. Deval Patrick will then appoint an interim senator and set the date of the special election, expected in early summer.

A primary race between Markey and Lynch would be a stark contrast in backgrounds and constituencies. Lynch represents a mostly urban district, which includes South Boston and has many middle- and working-class constituents who are Democratic but skew conservative. Markey represents a broad swath of suburbs outside Boston that includes some urban areas but also some of the wealthiest and most liberal municipalities in the state.

Lynch, who is anti-abortion and voted against the 2010 health care overhaul, will have the support of some local unions, while Markey, who supports abortion rights and voted for the health care overhaul, has the backing of the powerful Democratic Senatorial Campaign Committee and Kerry, among others.

Former Sen. Scott P. Brown, defeated by now-Sen. Elizabeth Warren in 2012, has not yet said whether he will run.