"Welcome to the NFL, kid," the coach of the Minnesota Vikings once remarked to a dazed rookie who had just been blind-sided by a blitzing linebacker for the first time. The same could be said to Rep. Timothy E. Wirth (D-Colo.), who staged a balloon-and-bunting rally in Denver yesterday to announce his candidacy for the Senate seat of Gary Hart (D-Colo.). Hart is stepping down next year, presumably to run for the Democratic presidential nomination in 1988.

Wirth, a six-term House member from the Boulder area with a generally liberal voting record, faces the same tough fight Hart would have had he sought reelection and inherits something else as well: a conservative former state legislator announced that the "Gary Hart Truth Squad" had just changed its name to the "Tim Wirth Truth Squad" and will spend about $200,000, mostly from the National Conservative Political Action Committee, on negative advertising against him.

One Republican criticism will be Wirth's role as chairman of the House subcommittee that oversaw the breakup of of American Telephone & Telegraph Co.

Wirth, however, has been a skillful campaigner in his marginal district. He won in 1984 by 53 to 46 percent while President Reagan carried his district by 61 percent. A Denver Post poll in mid-December showed Wirth tied with Rep. Ken Kramer (R-Colo.), the early favorite for the GOP nomination, with 41 percent each. But Kramer's campaign hit a snag this week when his campaign manager left.