It’s a heady time to be Virginia Gov. Bob McDonnell. As the Republican approaches the halfway mark of his term this winter, his approval rating last week was above 60 percent, or nearly 20 points higher than President Obama’s.

GOP presidential candidates Rick Perry and Mitt Romney compete for McDonnell’s potential endorsement. The Virginian recently took over as chairman of the Republican Governors Association. He is one of the two politicians most frequently mentioned as a possible vice presidential running mate, the other being Sen. Marco Rubio (R-Fla.).

McDonnell has earned his enviable position. So far, he has fulfilled the central promise of his 2009 campaign, which was to govern as a pragmatic conservative rather than an ideologue. He put together a strong Cabinet, has prudently managed the state’s finances and found a short-term fix for the commonwealth’s cash-strapped roads (albeit by borrowing the money).

Nevertheless, McDonnell cannot afford to coast right now. For one thing, the weak economy might be catching up with him. He ran on a pledge to boost employment — “Bob’s for jobs” was a campaign slogan — but the latest report Friday showed that Virginia has lost 24,500 jobs over the past four months.

Moreover, the true test of McDonnell’s administration, which the law limits to one term, is yet to come. In the next six months, he’ll face two major challenges that will help determine whether he goes into history as an above-average governor who performed well in tough times or a transformational figure who set Virginia on a firm conservative course for years to come.

The first trial comes in the November General Assembly elections, when McDonnell seeks to lead his party to wrest the Senate from Democratic control. That would put the GOP in full command of the legislature as well as the executive branch, since the Republicans are widely expected to hang onto the House of Delegates.

The GOP needs to pick up only two Senate seats to gain dominance, but even some Republicans here in Richmond say they’d be fortunate to do so. That’s largely because Senate Democrats redrew districts after the 2010 Census to protect their majority. (Republicans did the same in the House.)

The Senate result will have a big impact on how McDonnell fares in his second major challenge: crafting and selling an ambitious, conservative agenda for the legislative session beginning in January.

This is the session that counts most for McDonnell in his entire term. Because Virginia has a two-year budget cycle and a four-year governor, the budget he presents will be the only one that he develops from scratch and then implements.

A big splash is important also because McDonnell’s policy achievements so far have been either defensive, comparatively modest in scale or aided by budget gimmicks and federal stimulus funds.

For example, he balanced the budget without a tax increase in his first year in office, but he did so partly by shortchanging the Virginia public employees’ pension fund. He’s proud of investing more money in higher education, but the small sum was at best a down payment on what’s needed.

McDonnell doesn’t yet have a signature accomplishment that would define his term and — not coincidentally — look good in television ads if he landed on the national Republican ticket. His supporters say two consecutive budget surpluses qualify, but Democrats would instantly note how he benefited from cash from Washington and financial sleight of hand.

Moreover, given the slow economy and loss of federal stimulus funds, the governor is going to have to make hard spending choices just to balance the budget again.

What else could McDonnell deliver? He won’t make final decisions on his legislative proposals until December, but Republican officials and analysts here pointed to several likely targets.

A top priority will be overhauling the state pension fund, which is deeply in the red in the long term. Other goals will be doing more for colleges and universities, and possibly expanding merit pay for K-12 teachers.

McDonnell is also considering trying again to persuade the General Assembly to privatize state liquor stores to raise money for roads. Even if he succeeds — a big if — that would provide only temporary relief for the state’s strained transportation system.

McDonnell’s prospects looked bright Wednesday when he introduced Texas Gov. Perry to a large, enthusiastic crowd of Republican activists at a fundraiser at the Richmond convention center. When the two stood together, everyone in the house thought, “That could be the Republican ticket.”

Perry’s top rival for the nomination, former Massachusetts governor Romney, phoned McDonnell the same day to make sure the Virginian hadn’t formally endorsed the Texan.

McDonnell’s success or failure in the Senate campaign and the legislative session will go a long way to determine whether his star continues to brighten on the national scene or has achieved as much luster as it ever will.