How the mighty have fallen.

Nevada, California, and Florida have the nation’s weakest economies, according to a midyear review of state employment trends by Portfolio.com and bizjournals. (For the complete ranking of the 50 states and the District of Columbia, click here to download a pdf.)

That’s a stunning reversal from half a decade ago. Nevada and Florida finished first and second, respectively, in 2005’s midyear review. California was a respectable 11th.

But this year’s study puts Nevada in 51st place, dead last among the 50 states and the District of Columbia. California (50th) and Florida (49th) are barely a step ahead.

Several factors have turned the three leaders into laggards during the past five years, notably the deflation of their hyperexpanded real estate bubbles and a sharp downturn in tourism.

Nevada, California, and Florida have collectively lost 1.69 million jobs since 2005. All are currently saddled with double-digit unemployment rates, with Nevada the worst at 14.0 percent.

Surprises can also be found at the top of the new midyear standings. Tiny North Dakota enjoys the nation’s strongest economy at the moment, and Alaska holds second place, according to the Portfolio.com/bizjournals rankings.

Both front-runners registered impressive gains during the past half decade at a time when most other states were suffering sizable declines. North Dakota expanded its employment base by 7.8 percent from 2005 to 2010, adding 21,300 jobs. Alaska’s corresponding gain was 4.4 percent, or 10,100 jobs.

Portfolio.com and bizjournals have developed a nine-part formula to analyze state-by-state employment trends. The formula uses U.S. Bureau of Labor Statistics data for the latest five-year period (May 2005 through May 2010), focusing on raw and percentage changes in private-sector employment, as well as unemployment rates.

The recent half decade was not kind to most parts of the country. Forty states had fewer jobs in May 2010 than five years earlier. The nation lost a total of 4.51 million private-sector positions between mid-2005 and mid-2010.

But the severity of the economic recession has been tempered in states with affordable housing, especially those in the heartland that stretches from the Gulf of Mexico to the Canadian border.

Six of the top 10 states are located within that broad belt, including North Dakota (first place), Texas (third), South Dakota (fourth), Nebraska (sixth), Louisiana (seventh), and Utah (10th).

The other leaders, besides Alaska, come from the Northeast, which has weathered the recession with greater ease than the Sunbelt. The Northeastern members of the upper echelon are the District of Columbia (fifth), New York (eighth), and New Hampshire (ninth).

Seven of the 10 strongest states managed to add private-sector positions between 2005 and 2010. The top 10, as a group, gained a total of 485,600 jobs. Their collective unemployment rate was 7.9 percent as of May, with North Dakota posting the nation’s best rate, a microscopic 3.6 percent.

The 10 states at the bottom of the standings, on the other hand, were burdened with an overall unemployment rate of 11.8 percent in May. All 10 have suffered private-sector losses since 2005, resulting in a collective drop of 3.12 million jobs.

Several of the nation’s biggest states join California and Florida on this unhappy list, including Michigan (48th place), Georgia (46th), Ohio (45th), and Arizona (44th). Their declines have been triggered by varying combinations of real estate woes, slowdowns in the construction and tourism sectors, and slippage in manufacturing output, especially within the auto industry.

The employment bases of these bottom states have eroded at an alarming pace. California’s loss of 950,300 private-sector jobs since 2005 is the equivalent of losing 520 jobs every day for five years. Florida’s rate of decline is 350 jobs per day, and Michigan’s is 280 per day.

For the complete ranking of the 50 states and the District of Columbia, click here to download a pdf.