Dan Roberts can do the math.
The stockbroker from Tiburon, one of 12 candidates for the North Coast congressional seat, knows that support from fellow Republicans isn't enough in the June 5 primary election.
"I need 3 to 5 percent of the Democrats to come my way," he said.
Republicans are outnumbered more than 2-to-1 by Democrats among registered voters in the six-county 2nd Congressional District stretching from Marin to the Oregon border, and hold a base of less than 30 percent of the vote.
Roberts, a political newcomer who has loaned his campaign $160,000, probably had a chance at finishing second — and advancing to the November runoff election — until Michael Halliwell of Cotati, unsuccessful in three previous bids for Congress, registered for a fourth race.
In the 2010 Republican primary, Halliwell got 32 percent, limiting the winner, Jim Judd of Rohnert Park, to 68 percent.
Roberts, 69, a Vietnam War veteran, might appeal to moderate Democrats by advocating an immediate withdrawal from Afghanistan "to halt the loss of life and limb" and the "drain on our national treasury."
But he is otherwise in step with GOP orthodoxy, saying the nation "can't spend or borrow our way out of the malaise we're in" and advocating elimination of the Energy and Education departments and the Environmental Protection Agency.
If a liberal Democrat wins the North Coast seat, "we're going to live with them for years and years," he said.
Roberts and Halliwell are among the eight long-shot candidates in the race, which has no incumbent.