Bumper-to-bumper traffic westbound on I-66 in Vienna, Va. (Marvin Joseph/Washington Post)

Gov. Terry McAuliffe (D) has made a Democratic takeover of the closely divided state Senate his top priority this fall. For several months, he’s tried to convince voters that control of the chamber is the only way he’ll be able to pass his agenda, which includes expanding Medicaid and passing new gun-control legislation.

But this week, he’s been tangling on the radio with Republicans not about health care or firearms but congestion plans for one stretch of road inside the Capital Beltway.

Currently, rush-hour restrictions apply to single drivers on that busy section of Interstate 66; under the governor’s plan, single drivers could pay to join the other commuters. Republicans have seized on the issue, accusing the governor of planning a new toll road that would increase the cost of commuting. House Republicans more than doubled their original spending on TV commercials knocking the I-66 toll plan for a total ad buy of $850,000, according to Matt Moran, spokesman for Speaker William J. Howell (R-Stafford).

Tolls have quickly become the key issue in the state Senate race between Republican Harry J. “Hal” Parrish II and Democrat Jeremy McPike to fill the seat that retiring Sen. Charles J. Colgan (D-Prince William) has held for 40 years. The Democrats’ efforts to control the chamber largely hinge on winning the district, where voters use I-66 heavily to commute to and from Washington, Tysons Corner and Virginia’s inner suburbs.

The governor has highlighted this issue in consecutive morning radio interviews this week. On Wednesday, a frustrated McAuliffe defended the I-66 proposal, calling Parrish a liar engaging in “foolishness” for his criticism of the plan.

Parrish, the mayor of Manassas, responded Thursday with a letter to the governor, saying he was “surprised and disappointed” that McAuliffe would accuse him of lying. “I stand behind my position and reject your characterization of me,” Parrish wrote.

Despite McAuliffe’s insistence that his plan is the best way to reduce commuting times, Democrats, including McPike, have backed away from supporting his idea to give solo drivers permission to drive in the carpool lanes in I-66 in 2017. For a price. The administration also changed course on the part of the plan that would have imposed tolls on drivers traveling against morning and evening rush hours, known as reverse commuters.

In addition to tolls, which will probably help the race become one of the most expensive in state history — with about $4 million raised — the contest has attracted groups seeking tighter gun-control laws. About a quarter of that money comes from Everytown for Gun Safety, an advocacy group launched by former New York mayor Michael Bloomberg to help candidates who support stricter access to guns. Everytown is spending $1.5 million backing McPike. “You can’t turn on the TV for two minutes without seeing one of those ads,” said Senate Minority Leader Richard L. Saslaw (D-Fairfax).

But despite that flood, as well as the advocacy of the parents of a Roanoke television reporter slain on live television in August, Republicans say momentum is on their side.

“It feels good out there,” said Ed Gillespie, former Republican National Committee chairman who is considering a run for governor in 2017. “I think we’re going to expand our majority in the Senate. . . . We may even expand the majority in the House.”

Meanwhile, Democrats have touted what McAuliffe political director Brian Zuzenak called an “unprecedented campaign operation,” saying it’s knocked on more doors and made almost as many phone calls as Sen. Mark R. Warner (D) did in his statewide reelection bid last year.

“They’re spending a lot of money on turnout and everything else,” said former Virginia representative Tom Davis (R), who chaired the National Republican Congressional Committee. “But it’s hard to turn out people who don’t want to show up.”

The toll issue is being used in House races to hammer Del. Kathleen Murphy (D-Fairfax) as well as Democratic candidates John Bell and Jennifer Boysko, both in Fairfax County. Murphy faces a rematch against Craig Parisot, the Republican she narrowly defeated in a special election last year.

Howell contributed $315,000 from his political action committee to Parisot’s campaign in the past few days, accusing Murphy of backing the governor’s plan.

“It truly is the most dishonest effort. We’ve all come out strongly against I-66 tolls,” Murphy said. “It’s a crazy time.”

Howell has also given $300,000 to Danny Vargas (R), a businessman running against Boysko for the seat being vacated by Del. Thomas Davis Rust (R-Fairfax).

Vargas said that after knocking on thousands of doors, he’s heard Medicaid expansion come up “maybe a couple of times.”

Rust was the only House Republican to vote in favor of Medicaid expansion, one of McAuliffe’s top priorities. Vargas said he’s interested in finding a private-sector way to cover the uninsured, noting that he relied on Medicaid while growing up in Brooklyn.

The idea of tolls, Vargas said, “have come up quite a bit.”

But McAuliffe is intensely focused on the Senate, hoping to reclaim the votes he would need to pass a Medicaid expansion and force House Republicans to negotiate the issue. He’ll fly across Virginia on Sunday with Warner in hopes that their combined star power can get out in the vote in several key races, including for the seat Colgan is vacating.

“I think the governor decided he doesn’t care much about the House Democratic Caucus,” said state Del. Gregory D. Habeeb (R-Salem), who is leading GOP efforts. “I don’t think there’s a single race we’re playing in where we’re not spending more.”