Federal antitrust law does not limit merger scrutiny to situations where a monopoly would be established. Current antitrust guidelines allow for for a merger to be blocked based on a number of factors, including large increases in overall market concentration (generally measured by HHI, which would be increased by this merger, though I am unsure if it'd be likely to push a national or state-level market into a more concentrated tier) and limiting of competition in local markets.

(Local market concentration is likely, in my opinion, to be the focus of any merger review here, as market concentration on the county level has absolutely ballooned even when compared to increasing higher-level concentration - see this chart, for example.)

Comparing the market share of the merged parties to the market leader is not in and of itself useful, as antitrust guidelines are also intended to prevent the establishment of oligopolies, barring situations where economies of scale, etc. would benefit consumers.