Days after the Supreme Court upheld President Obama’s health-care law, WellPoint (WLP) inked a $4.5 billion transaction on Monday to scoop up rival health insurer Amerigroup (AGP) in an effort to bolster its Medicaid offerings.

WellPoint agreed to pay $92 a share for Virginia Beach-based Amerigroup, representing a solid 43% premium on the company’s Friday closing price. When debt is included, the valuation of the acquisition rises to $4.9 billion.

Explaining its decision to snatch up Amerigroup, WellPoint pointed to the company’s focus on the growing Medicaid population.

"We believe that this combination will create an industry leader in the government sector serving Medicaid and Medicare enrollees,” WellPoint CEO Angela Braly said in a statement.

The combined companies will have a Medicaid footprint in 19 states and 4.5 million beneficiaries of state-sponsored health programs.

The deal comes just days after the Supreme Court narrowly voted to uphold the health-care reform law, which is set to take full effect in 2014 and expand Medicaid availability to about 16 million Americans.

“This is an opportunity to capitalize on the strengths of both companies to better serve our members and position our companies for future growth as the health insurance industry changes,” said Braly.

WellPoint said it expects the takeover to boost its bottom line in 2013 and add more than $1.00 per share to earnings by 2015. The acquisition is seen closing in the first quarter of next year.

“Today’s health care arena is more transformative than ever and companies must have broad capabilities to succeed,” said Amerigroup CEO James Carlson.

Shares of Amerigroup soared 38.06% to $88.83 ahead of Monday’s opening bell, but traded below the $92 offer price. Other Medicaid-related stocks also rallied, including WellCare (WCG) and Centene (CNC), which leaped 12% each.

WellPoint also enjoyed solid gains, rising 4.74% to $62.75 in premarket action.

Credit Suisse (CS) served as a financial advisor on the deal for WellPoint, while Goldman Sachs (GS) and Barclays (BCS) advised Amerigroup.

Follow Matt Egan on Twitter @MattMEgan5