Using Census data, we've figured out that half of the United States population is clustered in just the 146 biggest counties out of over 3000.
Here's the map, with said counties shaded in and below it is the list of the counties.
Map of US 50 percent
Census/Business Insider
And here's the whole list of counties that are shaded in.
Los Angeles County, CA
Cook County, IL
Harris County, TX
Maricopa County, AZ
San Diego County, CA
Orange County, CA
Miami-Dade County, FL
Kings County, NY
Dallas County, TX
Queens County, NY
Riverside County, CA
San Bernardino County, CA
King County, WA
Clark County, NV
Tarrant County, TX
Santa Clara County, CA
Broward County, FL
Wayne County, MI
Bexar County, TX
New York County, NY
Alameda County, CA
Philadelphia County, PA
Middlesex County, MA
Suffolk County, NY
Sacramento County, CA
Bronx County, NY
Palm Beach County, FL
Nassau County, NY
Hillsborough County, FL
Cuyahoga County, OH
Allegheny County, PA
Oakland County, MI
Orange County, FL
Franklin County, OH
Hennepin County, MN
Fairfax County, VA
Travis County, TX
Contra Costa County, CA
Salt Lake County, UT
Montgomery County, MD
St. Louis County, MO
Pima County, AZ
Fulton County, GA
Honolulu County, HI
Mecklenburg County, NC
Westchester County, NY
Milwaukee County, WI
Wake County, NC
Fresno County, CA
Shelby County, TN
Fairfield County, CT
DuPage County, IL
Pinellas County, FL
Erie County, NY
Marion County, IN
Bergen County, NJ
Hartford County, CT
Prince George's County, MD
Duval County, FL
New Haven County, CT
Kern County, CA
Macomb County, MI
Gwinnett County, GA
Ventura County, CA
Collin County, TX
El Paso County, TX
San Francisco County, CA
Middlesex County, NJ
Baltimore County, MD
Pierce County, WA
Montgomery County, PA
Hidalgo County, TX
Worcester County, MA
Hamilton County, OH
Essex County, NJ
Multnomah County, OR
Essex County, MA
Jefferson County, KY
Monroe County, NY
Suffolk County, MA
Oklahoma County, OK
San Mateo County, CA
Snohomish County, WA
Cobb County, GA
Denton County, TX
DeKalb County, GA
San Joaquin County, CA
Lake County, IL
Will County, IL
Norfolk County, MA
Jackson County, MO
Bernalillo County, NM
Jefferson County, AL
Hudson County, NJ
Davidson County, TN
Lee County, FL
El Paso County, CO
Denver County, CO
District of Columbia, DC
Monmouth County, NJ
Providence County, RI
Fort Bend County, TX
Bucks County, PA
Baltimore city, MD
Polk County, FL
Kent County, MI
Tulsa County, OK
Arapahoe County, CO
Ocean County, NJ
Delaware County, PA
Johnson County, KS
Bristol County, MA
Anne Arundel County, MD
Washington County, OR
Brevard County, FL
New Castle County, DE
Jefferson County, CO
Union County, NJ
Summit County, OH
Utah County, UT
Montgomery County, OH
Douglas County, NE
Lancaster County, PA
Kane County, IL
Stanislaus County, CA
Ramsey County, MN
Camden County, NJ
Chester County, PA
Sedgwick County, KS
Dane County, WI
Passaic County, NJ
Guilford County, NC
Plymouth County, MA
Morris County, NJ
Volusia County, FL
Lake County, IN
Sonoma County, CA
Montgomery County, TX
Spokane County, WA
Richmond County, NY
Pasco County, FL
Greenville County, SC
Onondaga County, NY
Hampden County, MA
Adams County, CO
Williamson County, TX